The present invention relates generally to tape storage systems. More specifically, the present invention relates to a device and method for controlling the position of a head relative to a tape within a tape transport system, considering lateral tape motion and environmental vibrations.
In modern tape storage systems, the capacity and performance of the tape storage systems have increased considerably. However, to achieve higher cartridge or tape capacities and improved performance, further advances in several areas are necessary. Increases in linear and track densities on the tape can be required in order to achieve higher storage capacities. However, an increase in linear densities can lead to a decrease of the distance between adjacent bit cells, which in turn can lead to an increase of inter-symbol interferences. An increase in track densities can lead to narrower individual track widths and narrower write and/or read heads, which in turn can require very precise control of the tape transport system and track-follow control of the tape head.
To achieve the required track densities, precise positioning of the recording head over the data tracks can be required. Therefore, the performance of the track-follow control system of tape drives needs to be improved. Furthermore, operation of tape drive systems requires robust performance of the track-follow control system under shock and vibration conditions. Increasing the tape track density tightens further the tolerance in the acceptable track following error, making it increasingly more challenging to meet the performance specifications under vibration conditions.
The basic function of the track-follow control system is to reduce the misalignment between the tape and the recording head created by lateral motion of the flexible medium. Lateral tape motion (LTM) arises primarily from imperfections in the tape guide rollers and reels (e.g., such as run-outs, eccentricities, and other tape path imperfections).
Besides compensating for the LTM, the track-follow control system can provide an additional functionality of compensating for the external vibration disturbances. Conventionally, standard vibration profiles are used to describe the vibration specifications in terms of the acceleration input under which the tape drive must continue to operate reliably.
To accommodate different data rate requirements the tape drive needs to operate in a wide range of tape speeds. Operation at different tape speeds creates conflicting requirements in the track-follow control system. For example, the LTM disturbance frequency increases with tape speed. Therefore, to compensate for LTM at high speeds a higher bandwidth controller is needed to follow with the same accuracy. On the other hand the servo pattern delay increases with decreasing speed. In this case, a lower bandwidth controller is needed to avoid closed-loop disturbance amplification. Further, a high-bandwidth controller is needed to improve performance under vibration conditions at all speeds.
Current solutions address individual problems. For example, compensation of the high frequency LTM components can be achieved by increasing the controller bandwidth. With this approach though, there is a trade-off between the performance improvements by compensating the disturbance and the delay effects at low tape speeds. Several approaches have been proposed for improving performance under vibration condition such as switching controllers, accelerometer measurements, and disturbance observer enhancements. Controllers in this context refer to devices for controlling an actuator being responsible for actuating the head (i.e. to change the position of the head). For different vibration conditions, different controllers can be used (i.e. controllers having configurations customized to the current vibration condition). Depending on the vibration condition, the control signal for the actuator can be generated and adjusted so that the vibration condition is considered when actuating the head. However, in order to consider different vibration conditions, the kind of controller can be chosen. A system using switching between controllers is for example disclosed in US 2012/0307391 A1.